Sentence:  “I used to be a once bitten, twice shy kind of guy. Now I get hit by a <WSD>Mack truck</WSD> of a girl and realize my life is flashing in front of my eyes. Everything's going too fast anyway, seems easier to go jump in and go with the flow.” 
Meaning of the word marked with <WSD> tags:  Any unstoppable or enormous thing coming through.