A locking nipple fitting assembly (10; 90) for selective removable coupling with a housing having a mounting location for receiving the nipple fitting assembly, the assembly comprising: a base (22; 100) having first engagement structure (56; 128) configured to engage structure adjacent the mounting location such that the base is secured against rotation relative to the housing; a nipple fitting (20; 92) received on the base and having first and second oppositely disposed coupling features, the first coupling feature configured to mate with a complementary coupling feature of the housing at the mounting location whereby the nipple fitting may be secured to the housing; locking structure (62, 64; 110, 120) between the base and the nipple fitting, the locking structure configured to resist relative rotation between the base and the nipple fitting, wherein a) the nipple fitting is rotatable relative to the base such that the nipple fitting is operatively couplable with the housing in a fluid-tight manner, and the locking structure is configured such that its rotation resistance takes places when a torque applied to rotate the nipple fitting is below a threshold torque, preferably the locking structure comprising first locking structure (62; 110) associated with the base and second locking structure (64; 120) associated with the nipple fitting, said first and second locking structure cooperating to resist the relative rotation between the base and the nipple fitting, preferably said first locking structure and/or said second locking structure being configured as a resilient element that deforms upon relative rotation between the base and the nipple fitting; and/or b) the locking structure comprises first locking structure (62; 110) associated with the base and second locking structure (64; 120) associated with the nipple fitting, said first and second locking structure cooperating to resist the relative rotation between the base and the nipple fitting, wherein said first locking structure and/or said second locking structure is configured as a resilient element that deforms upon relative rotation between the base and the nipple fitting. a base (22; 100) having first engagement structure (56; 128) configured to engage structure adjacent the mounting location such that the base is secured against rotation relative to the housing; a nipple fitting (20; 92) received on the base and having first and second oppositely disposed coupling features, the first coupling feature configured to mate with a complementary coupling feature of the housing at the mounting location whereby the nipple fitting may be secured to the housing; locking structure (62, 64; 110, 120) between the base and the nipple fitting, the locking structure configured to resist relative rotation between the base and the nipple fitting, wherein a) the nipple fitting is rotatable relative to the base such that the nipple fitting is operatively couplable with the housing in a fluid-tight manner, and the locking structure is configured such that its rotation resistance takes places when a torque applied to rotate the nipple fitting is below a threshold torque, preferably the locking structure comprising first locking structure (62; 110) associated with the base and second locking structure (64; 120) associated with the nipple fitting, said first and second locking structure cooperating to resist the relative rotation between the base and the nipple fitting, preferably said first locking structure and/or said second locking structure being configured as a resilient element that deforms upon relative rotation between the base and the nipple fitting; and/or b) the locking structure comprises first locking structure (62; 110) associated with the base and second locking structure (64; 120) associated with the nipple fitting, said first and second locking structure cooperating to resist the relative rotation between the base and the nipple fitting, wherein said first locking structure and/or said second locking structure is configured as a resilient element that deforms upon relative rotation between the base and the nipple fitting. a) the nipple fitting is rotatable relative to the base such that the nipple fitting is operatively couplable with the housing in a fluid-tight manner, and the locking structure is configured such that its rotation resistance takes places when a torque applied to rotate the nipple fitting is below a threshold torque, preferably the locking structure comprising first locking structure (62; 110) associated with the base and second locking structure (64; 120) associated with the nipple fitting, said first and second locking structure cooperating to resist the relative rotation between the base and the nipple fitting, preferably said first locking structure and/or said second locking structure being configured as a resilient element that deforms upon relative rotation between the base and the nipple fitting; and/or b) the locking structure comprises first locking structure (62; 110) associated with the base and second locking structure (64; 120) associated with the nipple fitting, said first and second locking structure cooperating to resist the relative rotation between the base and the nipple fitting, wherein said first locking structure and/or said second locking structure is configured as a resilient element that deforms upon relative rotation between the base and the nipple fitting.