Sorry about the links, if you have a library card from England, Scotland, or Wales your library most likely subscribes and you should be able to login using your library card number, outside of those countries there's a chance your library subscribes and you may be able to login but probably not. I did try to find an entry on Wiktionary but couldn't find one, although this time I found an entry on another dictionary that does appear to be accessible:

https://quod.lib.umich.edu/m/middle-english-dictionary/dicti...

The quotations on the OED are in a similar Middle English time period from 950 to 1325, and the forms the OED lists are as follows:

Forms: Old English–Middle English wǽpman, wǽpnman, wǽpen-, wépenmon, Middle English wepman, ( Orm. weppmann), weopmonne, wepmon(ne, wapmon, wapman.

Etymology: Old English wǽpnman , < wǽpn weapon n. (= membrum virile ) + man man n.1 Compare Old English wǽpned adjective, male.

So it likely comes from 'wæpnedmann' but I couldn't find anything explicitly for that (or variations on spelling), maybe OE is a little too old for the OED or as you say maybe it just isn't that well attested, the closest I could find was the following:

OE tr. Alexander's Let. to Aristotle (1995) §29. 242 Ða gesawe we þær ruge wifmen, & wæpned men wæron hie swa ruwe & swa gehære swa wildeor [L. pilosos in modo ferarum toto corpore].