means Words, ideas, or behavior that make no sense, carry no meaning, or are foolishly absurd.

from A transparent compound: the prefix "non-" (not, from Latin "non") bolted onto "sense" (meaning, perception, from Latin "sensus," "feeling" or "understanding," via Old French). So at its root it's simply "not-sense" — the absence of meaning — an English coinage that has been around since roughly the early 17th century. Its French cousin "nonsens" follows the very same logic, and English later coined fancier siblings like "nonsensical."