Slip Into Something More Comfortable Meaning

(idiomatic) To wear something suitable to be stripped off by a lover.

Example: 2009, Carl Weber, Mary B. Morrison, She Ain't the One
  Pointing to my bedroom, I said, “The bathroom's upstairs.” “Aw'ight, chill. You act like you got Tupac in there or something.” Jay laughed, then said, “I'll meet you upstairs. Why don't you slip into something a little more comfortable?"