let on

HUCKLEBERRY FINNが女の子の衣装を着て女の子のふりをして知らない女の人と話をしている場面にあった慣用句です。
When the woman stopped talking I looked up, and she was looking at me pretty curious and smiling a little. I put down the needle and thread, and let on to be interested -- and I was, too -- and says:
"Three hundred dollars is a power of money. I wish my mother could get it. Is your husband going over there to-night?"
この "let on" は文脈からして、興味をもった「ふりをする」意味だと推測できますが、知らない慣用句だったので辞書で確認します。
・Dictionary.com:
a.to reveal one's true feelings: She was terrified at the prospect, but didn't let on.
b.to pretend: They let on that they didn't care about not being invited, but I could tell that they were hurt.
・Merriam-Webster's Online Dictionary:
1: to make acknowledgment: admit (knows more than he lets on)
2: to reveal a secret (nobody let on about the surprise party)
3: pretend (let on to being a stranger)
ふりをするのと、口外する、暴露するでは正反対に近い意味になる場合もあるのではと思います。　ふりをする場合は "let on to" となる場合が多いのかなとも思いましたが、ふりをするで "let on that" の形を取るのは他にも次ぎのEncartaにあった例の様に多く見つかるので、どちらの意味かの判断は文脈でするしかないようです。
・pretend: to make somebody believe something that is not true
She let on that she was upset, but she wasn't really that bothered.
・secret: to share a secret with somebody ( informal )
He didn't let on that he was very rich.