Proto-Indo-European root meaning "to lean."
It forms all or part of: acclivity; anticline; clemency; client; climate; climax; cline; clinic; clinical; clino-; clitellum; clitoris; decline; declivity; enclitic; heteroclite; incline; ladder; lean (v.); lid; low (n.2) "small hill, eminence;" matroclinous; patroclinous; polyclinic; proclitic; proclivity; recline; synclinal; thermocline.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit srayati "leans," sritah "leaning;" Old Persian cay "to lean;" Lithuanian šlyti "to slope," šlieti "to lean;" Latin clinare "to lean, bend," clivus "declivity," inclinare "cause to bend," declinare "bend down, turn aside;" Greek klinein "to cause to slope, slant, incline;" Old Irish cloin "crooked, wrong;" Middle Irish cle, Welsh cledd "left," literally "slanting").
"fireman's scaling ladder," short for pompier ladder (by 1893), French, literally "fireman," from pompe "pump" (see pump (n.1)).
1650s, "resembling a ladder," from Latin scalaris "of or pertaining to a ladder," from scalae (plural) "ladder, steps, flight of steps" (see scale (n.2)). The noun in the mathematical sense of "a real number" is from 1846, coined by Irish mathematician William R. Hamilton (1805-1865), who can explain why it is the correct word for that.
"to climb (a wall) by or as by a ladder; attack with scaling ladders," late 14c., scalen, from Latin scala "ladder, flight of stairs," from *scansla, from stem of scandere "to climb" (see scan (v.)).
Middle English scale, "ladder used in sieges," is attested c. 1400, from the Latin noun. The verb in general and figurative use (of mountains, heights of pleasure, etc.) is from 16c.
Via scale (n.3) "standard of measure or estimation" comes the meaning "measure or regulate by a scale" (1798), the sense of "draw, project, or make according to scale" (by 1885), and scale down "cut or decrease proportionally in every part" (by 1887). Related: Scaled; scaling.