日本語 Japanese Ask Questions Thread 8

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英語でも日本語でもOK
日本の方からのご協力も大歓迎です

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達人かと思うだろ or more generally just ~と思うだろう is "One would think ~ (if you do something that came before)". He's talking about himself here, so it's "(if you compare on the level of millimeters) I'll start thinking you're some kind of expert"

The millimeters comes because she's splitting hairs about whether he touched her clothes or her tits, which is something no one can really tell (unless you are 達人 and you know exactly how far his hands went, down to the millimeter).

It's never going to be "don't act like you know what I have up my sleeve" (I don't really know where you're getting that from) and it's not saying anything about what she thinks of herself.