Jackies for Sailors. Vulgar, and especially offensive to seamen.
Jeopardize for Imperil, or Endanger. The correct word is jeopard, but in any case there is no need for anything so farfetched and stilted.
Juncture. Juncture means a joining, a junction; its use to signify a time, however critical a time, is absurd. "At this juncture the woman screamed." In reading that account of it we scream too.
Just Exactly. Nothing is gained in strength nor precision by this kind of pleonasm. Omit just.
Juvenile for Child. This needless use of the adjective for the noun is probably supposed to be humorous, like "canine" for dog, "optic" for eye, "anatomy" for body, and the like. Happily the offense is not very common.