"mood employed to denote an action or state as conceived and not as a fact," 1620s, from earlier adjectival use of subjunctive (1520s), from Late Latin subiunctivus "serving to join, connecting," from subiunct-, past participle stem of Latin subiungere "to append, add at the end, place under," from sub "under" (see sub-) + iungere "to join together" (from nasalized form of PIE root *yeug- "to join"). The Latin modus subiunctivus probably is a loan-translation by the grammarians of Greek hypotaktike enklisis "subordinated," so called because the Greek subjunctive mood is used almost exclusively in subordinate clauses.
Proto-Indo-European root meaning "to join."
It forms all or part of: adjoin; adjust; conjoin; conjugal; conjugate; conjugation; conjunct; disjointed; enjoin; injunction; jugular; jostle; joust; join; joinder; joint; jointure; junction; juncture; junta; juxtapose; juxtaposition; rejoin (v.2) "to answer;" rejoinder; subjoin; subjugate; subjugation; subjunctive; syzygy; yoga; yoke; zeugma; zygoma; zygomatic; zygote.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit yugam "yoke," yunjati "binds, harnesses," yogah "union;" Hittite yugan "yoke;" Greek zygon "yoke," zeugnyanai "to join, unite;" Latin iungere "to join," iugum "yoke;" Old Church Slavonic igo, Old Welsh iou "yoke;" Lithuanian jungas "yoke," jungti "to fasten to a yoke;" Old English geoc "yoke."
1846, toast or expression wishing good health (from 16c., famously a drinking pledge by German students), Latin, literally "may it advantage (you)," third person singular present subjunctive of prodesse "to do good, be profitable" (see proud (adj.)).
direction to printer to disregard correction made to text, 1755, from Latin stet "let it stand," third person singular present subjunctive of stare "to stand, stand upright, be stiff" (from PIE root *sta- "to stand, make or be firm").
"warning, hint of caution," 1550s, Latin, literally "let him beware," 3rd person singular present subjunctive of cavere "to beware, take heed, watch, guard against," from PIE root *keu- "to see, observe, perceive." Legal sense "public warning preventing some action" is from 1650s.