[Jarmsy]

But common in informal English as a shortening of confesses.

[wiredfool]

I've only seen that as 'fess up'

[wizzwizz4]

In my head, it's a conjugation of "fess", which is a shortening of "confess". (Until I thought of it that way I was thinking "I've never seen this before", despite the word being in my vocabulary.)