When you don't know what gift to buy someone at Christmas time, so instead you just give them some money so they can buy something themself. Christmas cash is also a song by the hit youtuber named Fred.
Theodore: Oh crap dude, I don't know what to buy my girlfriend for a Christmas present!
Samuel: Just get her some Christmas cash.
Fred: Caaaash! Cash money! Money! All I really want is caaaash! Cash money!