penthouse (n.)

late 14c., appenden, "to belong to as a possession or right," from Old French apendre (13c.) "belong, be dependent (on); attach (oneself) to; hang, hang up," and directly from Latin appendere "cause to hang (from something); weigh out," from ad "to" (see ad-) + pendere "to hang, cause to hang; weight; pay" (from PIE root *(s)pen- "to draw, stretch, spin").

The meaning "to hang on, attach as a pendant" is by 1640s; that of "attach as an appendix" is recorded by 1843. OED says the original word was obsolete by c. 1500, and these later transitive senses thus represent a reborrowing from Latin or French. Related: Appended; appending.

Proto-Indo-European root meaning "to draw, stretch, spin."

It forms all or part of: append; appendix; avoirdupois; compendium; compensate; compensation; counterpoise; depend; dispense; equipoise; expend; expense; expensive; hydroponics; impend; painter (n.2) "rope or chain that holds an anchor to a ship's side;" pansy; penchant; pend; pendant; pendentive; pending; pendular; pendulous; pendulum; pension; pensive; penthouse; perpendicular; peso; poise; ponder; ponderous; pound (n.1) "measure of weight;" prepend; prepense; preponderate; propensity; recompense; span (n.1) "distance between two objects;" span (n.2) "two animals driven together;" spangle; spanner; spend; spider; spin; spindle; spinner; spinster; stipend; suspend; suspension.

It is the hypothetical source of/evidence for its existence is provided by: Latin pendere "to hang, to cause to hang," pondus "weight" (perhaps the notion is the weight of a thing measured by how much it stretches a cord), pensare "to weigh, consider;" Greek ponos "toil," ponein "to toil;" Lithuanian spendžiu, spęsti "lay a snare;" Old Church Slavonic peti "stretch, strain," pato "fetter," pina "I span;" Old English spinnan "to spin," spannan "to join, fasten; stretch, span;" Armenian henum "I weave;" Greek patos "garment," literally "that which is spun;" Lithuanian pinu "I plait, braid," spandau "I spin;" Middle Welsh cy-ffiniden "spider;" Old English spinnan "draw out and twist fibers into thread," spiðra "spider," literally "spinner."

"building whose rafters lean against another building or wall," mid-15c., from lean (v.) + to (adv.). Compare penthouse. "An addition made to a house behind, or at the end of it, chiefly for domestic offices, of one story or more, lower than the main building, and the roof of it leaning against the wall of the house" [Bartlett].