fess up

Mizuki Tsujimuraの 'Lonely Castel in the Mirror' を読んでいます。

'A proposal?'
Masamune and Aki exchanged a look.
'The other day, the two of us just happened to be alone in the castle and we started talking about it. I'll fess up -- Masamune and I searched very hard for the key. We looked everywhere.'

"fess up" を調べます。
・Oxford English Dictionary: (informal) to admit that you have done something wrong: It’s time for the President to fess up.:
・Cambridge English Dictionary: to admit that you have done something that someone else will not like: Fess up - it was you who ate that last piece of cake, wasn't it?
・Merriam-Webster: to admit that one has done something wrong : confess: At first he denied everything, but eventually he fessed up.

"fess" は "confess" から来た俗語ですね。