A merlin (or merlyn) derived from the Middle English colloquialism for Falco columbarius, a small, bald falcon, a merlin is a vulgar maneuver, most often delivered from above--hence, the word's avian origins.
While not necessarily practiced--much like counterparts "the Cleavland steamer" and "space docking"--it has evolved into both urban myth and popular in-joke among groups of friends. There is much debate on what the act entails, often falling into regional categories. Thus, in common parlance it has become a stand-in for all bizarre, deviant and lascivious intimacies: a sort of profane catch-all, or ribald signifier, whereas what is signified is fluid and varies greatly with person and context.
The etymology of the merlin is also disputed, with some cultures ascribing it to the wizard, Merlin, of Arthurian Legend. This may be on account of the obscure and mysterious nature of the term and what it may, or may not, imply. The word can be found in literature dating as far back as the 16th century.
Currently, the growing reemergence and popularity of the term has lead, perhaps indirectly, to a decline in the popularity of its use as a given name.
be careful today. i have a feeling elizabeth's going to try to give you a merlin when you're sleeping.