is an expression in Japanese that is used to show that someone is ‘pretending to be (A)’, ‘putting on airs of (A)’, or ‘acting as if (A)’. This construction comes from a combination of the noun 振（ふ）り ‘a shake’, the 副助詞（ふくじょし） (adverbial particle) を
, and する
. It can be thought of as meaning ‘to give off the impression of (A)’, or ‘to shake off the impression of (A)’.
As ふり itself is a noun, this expression may be used with the 連体形（れんたいけい） (attributive form) of any other word.
I heard that this snake pretends to be dead when humans and bigger animals approach it.
When I was a child, I would often pretend to sleep.
This is not what I wanted, but since they went out of their way and bought it for me, I will pretend that I am happy.
You don't have to pretend that you are happy in front of me.
Because Kakeru-kun likes frogs, he always pretends to be a frog when he plays.
The noun 振（ふ）り has several different nuances in Japanese, with only one of them being used in this particular expression. That meaning is ‘to show a certain behavior pattern on the outside’. Due to this, ふり doesn’t specifically mean ‘to pretend’, just that what is happening on the outside may be different to what is happening within a person (either mentally or physically).