1560s, "chosen on account of special excellence or fitness," from Latin selectus, past participle of seligere "choose out, single out, select; separate, cull," from se- "apart" (see se-) + legere "to gather, select" (from PIE root *leg- (1) "to collect, gather").

"Carefully picked," hence "choice, of special excellence" (by 1580s). Related: Selectly; selectness. The noun meaning "a selected person or thing, that which is choice" is recorded from c. 1600.

1825, "the use of a coach as a public conveyance;" 1849 as "special instruction or training for an exam or an athletic contest;" verbal noun from coach (v.). 

see set (n.1).

The extra t is an arbitrary addition in various technical senses, from a lawn-tennis to a granite set. Each class of persons has doubtless added it to distinguish the special sense that means most to it from all others ; but so many are the special senses that the distinction is now no more distinctive than an Esq. after a man's name, & all would do well to discard it. [Fowler]

1660s, in medicine, "branch of pathology concerned with the body's symptoms;" from semiotic; also see -ics. The meaning "study or doctrine of signs and symbols with special regard to function and origin" is by 1880. 

"witches' sabbath," a midnight meeting supposed to have been held annually by demons, sorcerers, and witches under the leadership of Satan, to celebrate their orgies, 1650s, a special application of the French form of Sabbath (q.v.).

"of no legal force," 1630s, from special use of Latin invalidus "not strong, infirm, impotent, feeble, inadequate," from in- "not" (see in- (1)) + validus "strong" (from PIE root *wal- "to be strong").

plural of stirps, Latin, literally "stem, stalk, trunk of a plant," figuratively "scion, offspring, descendant; source, origin, foundation, beginning," a word of uncertain origin. Hence stirpiculture "breeding of special stocks or strains."

in Catholicism, "devotions consisting of special prayers or services on nine successive days," 1745, from Medieval Latin novena, fem. of Latin novenus "ninefold," from novem "nine" (see nine). 

"word coined for a special occasion," and not likely to be wanted again, 1884, from nonce "for a particular purpose" + word (n.). Said to be a translation of Littré's term mot d'occasion.

1703, in ethics, "the solution of special problems of conscience by application of general principles or theories;" see casuist + -ry. Even in the earliest printed uses the sense was pejorative.