is that a thing to borrow a phat keep sake and give it back

Hello,

Since this is a question you should be posting this in the general runescape section.

To answer you question:

Most unlent items equippable in the head, cape, neck, body, legs, gloves, main hand, off-hand, or boots slots can be used with the key as long as the player has the requirements to wear them.

Source:
https://runescape.wiki/w/Keepsake_key

if you keepsake a phat then it gets locked in your costume interface untill you un-keepsake it.

so to keepsake it you have to own the hat and cannot used a lent or borrowed hat