c. 1200, "fact of being born;" mid-13c., "act of giving birth, a bringing forth by the mother, childbirth," sometimes in Middle English also "conception;" also "that which is born, offspring, child;" from a Scandinavian source such as Old Norse *byrðr (replacing cognate Old English gebyrd "birth, descent, race; offspring; nature; fate"), from Proto-Germanic *gaburthis (source also of Old Frisian berd, Old Saxon giburd, Dutch geboorte, Old High German giburt, German geburt, Gothic gabaurþs), from PIE *bhrto past participle of root *bher- (1) "to carry; to bear children" (compare bear (v.)).
Suffix -th is for "process" (as in bath, death). Meaning "condition into which a person is born, lineage, descent" is from c. 1200 (also in the Old English word). In reference to non-living things, "any coming into existence" is from 1610s. Birth control is from 1914; birth certificate is from 1842.
1570s, "to hurt, damage;" 1580s, "fail become deficient" (senses now obsolete); 1590s, "to desert, revolt," from Latin defectus, past participle of deficere "to fail, desert," from de "down, away" (see de-) + combining form of facere "to do, make" (from PIE root *dhe- "to set, put"). Related: Defected; defecting.
early 15c., "want or lack of anything," especially lack of something essential to perfection or completeness, from Old French defect and directly from Latin defectus "failure, revolt, falling away," noun use of past participle of deficere "to fail, desert," from de "down, away" (see de-) + combining form of facere "to do, make" (from PIE root *dhe- "to set, put").