climax (n.) Look up climax at Dictionary.com
1580s, in the rhetorical sense (a chain of reasoning in graduating steps from weaker to stronger), from Late Latin climax (genitive climacis), from Greek klimax "propositions rising in effectiveness," literally "ladder," from root of klinein "to slope," from PIE root *klei- "to lean" (see lean (v.)).

The rhetorical meaning evolved in English through "series of steps by which a goal is achieved," to "escalating steps," to (1789) "high point of intensity or development," a usage credited by the OED to "popular ignorance." The meaning "sexual orgasm" is recorded by 1880 (also in terms such as climax of orgasm), said to have been promoted from c.1900 by birth-control pioneer Marie Stopes (1880-1958) and others as a more accessible word than orgasm (n.).
climax (v.) Look up climax at Dictionary.com
1835, "to reach the highest point," from climax (n.). Related: Climaxed; climaxing.