He becomes 'diffident' [='shy', 'timid'] around girls.
She was 'diffident' about stating her opinion.
She has a 'diffident' [='reserved'] manner.
(..) The end of the scene. Um, l ask you when you're telling everyone, and you say-- Um, tomorrow will be soon enough. And then l-- Right. Okay. Say, who was that rather diffident chap l saw you talking to round the back ? No one. Just some guy from the past. l-- lt's a bit of an awkward situation. (..)1999 Notting Hill