mid-15c., originally in the grammatical sense," from Latin coniunctivus "serving to connect," from coniunctus, past participle of coniungere "to join together," from assimilated form of com "with, together" (see con-) + iungere "to join together" (from nasalized form of PIE root *yeug- "to join").
The conjunctive mode (Late Latin coniunctivus modus) is the mode which follows a conditional conjunction or expresses contingency; more commonly it is called subjunctive. Non-grammatical sense of "connecting, uniting, solidifying" is from late 15c. Meaning "closely connected" is from c. 1600. Related: Conjunctively.