bed-wetting (n.) "involuntary urination while sleeping," 1844, from bed (n.) + present participle of wet (v.). Related: Bed-wetter.
wet (v.) Old English wætan "to wet, moisten, water; be or become wet;" see wet (adj.). From mid-15c. as "to intoxicate" (oneself). Meaning "urinate" is by 1925. Related: Wetted; wetting.