  The couplings between the neutrinos and exotic fermion can be probed in both
neutrino scattering experiments and dark matter direct detection experiments.
We present a detailed analysis of the general neutrino interactions with an
exotic fermion and electrons at neutrino-electron scattering experiments. We
obtain the constraints on the coupling coefficients of the scalar,
pseudoscalar, vector, axialvector, tensor and electromagnetic dipole
interactions from the CHARM-II, TEXONO and Borexino experiments. For the
flavor-universal interactions, we find that the Borexino experiment sets the
strongest bounds in the low mass region for the electromagnetic dipole
interactions, and the CHARM-II experiment dominates the bounds for other
scenarios. If the interactions are flavor dependent, the bounds from the
CHARM-II or TEXONO experiment can be avoided, and there are correlations
between the flavored coupling coefficients for the Borexino experiment. We also
discuss the detection of sub-MeV DM absorbed by bound electron targets and
illustrate that the vector coefficients preferred by XENON1T data is allowed by
the neutrino experiments.
