: one that does or has done something only once.

What does it mean to recon something?

verb. reconned; reconning; recons. Definition of recon (Entry 2 of 2) transitive verb. : to conduct a preliminary and especially an exploratory survey of : to do a reconnaissance of (something) In another village we sent an advance party to recon the place.—

Is Oncer a Scrabble word?

No, oncer is not in the scrabble dictionary.