Because the meaning of words expands over many generations, the thought process is not necessarily logical, but a little imagination helps us to understand how words progress from one meaning to the next. Adjacent appears to be related to jet through the Latin iacere:
early 15c., from Latin adiacentem (nominative adiacens) "lying at,"
present participle of adiacere "lie at, border upon, lie near,"
from ad- "to" (see ad-) + iacere "to lie, rest," literally "to
throw"
(see jet (v.))
early 15c., "to prance, strut, swagger," from Middle French jeter
"to throw, thrust," from Late Latin iectare, abstracted from
deiectare, proiectare, etc., in place of Latin iactare "toss about,"
frequentative of iacere "to throw, cast," from PIE root *ye-
"to do" (cognates: Greek iemi, ienai "to send, throw;" Hittite ijami
"I make"). Meaning "to sprout or spurt forth" is from 1690s.
etymonline.com emphasis mine
Among other things, the prefix ad- means toward and near:
word-forming element expressing direction toward or in addition to,
from Latin ad "to, toward" in space or time; "with regard to, in
relation to," as a prefix, sometimes merely emphatic, from PIE *ad-
"to, near, at" (cognate with Old English æt; see at).
etymonline.com emphasis mine
Conclusion:
Once someone throws something inanimate, it just sits there. Many years ago, my great-great-grandfather threw that big pile of rocks toward the old oak tree. Under his control, the rocks flew to the tree. After all these years, the pile of rocks still sits there--adjacent to the old oak tree.
