The sign for "bank" is usually spelled. Or maybe I should
say it is "sort of" spelled. The concept of "bank" (as in "The bank
approved my loan" is usually expressed as lexicalized fingerspelling, which is to say,
a few letters letters get a bit blended together, dropped, or only partially formed.
Just practice spelling the word until you can do it "very" quickly.
Also see: "TELLER"