pastie: /pay'stee/ n. An adhesive-backed label designed to be attached to a key on a keyboard to indicate some non-standard character which can be accessed through that key.

:pastie: /pay'stee/ n. An adhesive-backed label designed to be
attached to a key on a keyboard to indicate some non-standard
character which can be accessed through that key. Pasties are
likely to be used in APL environments, where almost every key is
associated with a special character. A pastie on the R key, for
example, might remind the user that it is used to generate the rho
character. The term properly refers to nipple-concealing devices
formerly worn by strippers in concession to indecent-exposure
laws; compare {tits on a keyboard}.
-- The AI Hackers Dictionary