You probably heard it from David Chappelle's Netflix special "The Closer".
Dyke: a word used to refer to lesbians; a term that lesbians use in gay circles; originally a derogatory term for lesbians, now often used with pride.
Frumpy: A female with lack of concern for appearance; clothing style that usually consists of bagging; careless slothing; no style; considerably out of shape.
So, a ''frumpy dyke'' is ''a lesbian with lack of concern for appearance, from clothes to body shape''.
'I would consider myself a feminist, and I didn't even know that at the time. All these years I thought it meant frumpy dyke'.
*Lisbon and Rigsby get on the elevator*
Jane: Oh, damn. I forgotsomething. Uh, I’ll meet you there.
Lisbon: Forgot what?
*elevator dings*
Jane: Uh. My, uh, frumbahollymorgansterning.
The process of going out with an attractive girl, making her your girlfriend, making her fat and ugly, then breaking up with her. This probably needs to be done by a fairly attractive and rich man. To frump the girl, he would need to get them to eat more but workout less.