mid-14c., "having a defect or flaw of any kind, inferior, in bad condition," from Old French défectif (14c.) and directly from Late Latin defectivus "imperfect," from defect-, past-participle stem of deficere "to desert, revolt, fail," from de "down, away" (see de-) + combining form of facere "to do, make" (from PIE root *dhe- "to set, put").
[D]efective generally takes the sense of lacking some important or essential quality; deficient, that of lacking in quantity: as defective teeth, timber, character; deficient supplies, means, intellect. The same difference is found between deficiency and defectiveness. [Century Dictionary]
In grammar, "wanting some of the usual forms of declension or conjugation" (late 15c.). Used in the sense of "mentally ill" from 1854 to c. 1935; as a noun, "person wanting in some physical or mental power," by 1869. Related: Defectively; defectiveness.
"conference, conversation, speech," especially with an enemy, mid-15c., parlai, from Old French parlée, from fem. past participle of Old French parler "to speak" (11c.), from Vulgar Latin *paraulare, from Late Latin parabolare "to speak (in parables)," from parabola "speech, discourse," from Latin parabola "comparison" (see parable).
"having the power of expressing strong emotions in vivid and appropriate speech; able to utter moving thoughts or words," late 14c., from Old French eloquent, from Latin eloquentem (nominative eloquens) "speaking, having the faculty of speech; eloquent," present participle of eloqui "to speak out" (see eloquence). Related: Eloquently.
1550s, in grammar, "to state the part of speech of a word or the words in a sentence," a verbal use of Middle English pars (n.) "part of speech" (c. 1300), from Old French pars, plural of part "a part," from Latin pars "a part, piece" (from PIE root *pere- (2) "to grant, allot") in the school question, Quae pars orationis? "What part of speech?" Transferred (non-grammatical) use is by 1788. Pars also was a common plural of part (n.) in early Middle English. Related: Parsed; parsing.