Green’s Dictionary of Slang

william n.4

[abbrev. of William, wm plays on W.M. = ways and means]

(US teen) a date who accompanies for the sake of having someone to be with; no relationship is involved.

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[US]Fayette Co. Leader (IA) 27 June 2/4: A William — A boy you date just for the sake of having a date. ‘William’ stands for ‘W.M.’ — and ‘W.M.’ stands for ‘Ways and means’. See?