A backronym treats an already existing word as an acronym and expands its letters into the words of a phrase, and so is effectively an acrostic. The word is a portmanteau of back and acronym. Backrony
Humorous example of a backronym used by NASA to name a treadmill on the International Space Station, in reference to American comedian Stephen Colbert
A warrant canary, posted in a Craftsbury, Vermont library in 2005: "The FBI has not been here (watch very closely for removal of this sign)". Announcing the receipt of a national security letter would violate the associated gag order, while removing the sign would not.