Wasn't is the contracted form of was not. But wanst/wans't was an archaic adverb meaning once:
"Be the bye, I wanst knew art ould woman of that name. She was my darling Tibbie, but a notorious drunkard". 1
"Now, shut your eyes, and turn round wanst" whispered the Grey Man. Shawn did as he was desired; but, when he looked about, he was struck all of a hape to find himself standing in his own bawn.... 2
1 1863, Roger Quinn, The Heather Lintie: Being Poetical Pieces, Spiritual and Temporal..., page 167
2 1838, Charles Dickens et al, Bentley's Miscellany, page 308
NB: I originally came across the contraction wans't (sic) here, which led me to wanst. So I'm wondering if there is any connection between wanst or wasn't or even wans't.
For example, sha'n't was the dated spelling of shan't, but shalln't is the archaic form of shan't. Are there cases where the apostrophe was missed out but was intended to mean "shall not"? (shant and shallnt)
Instances where wans't is used in place of wasn't (though this is rare):