Meaning

noun projecting horizontal beam fixed at one end only

verb project as a cantilever

verb construct with girders and beams such that only one end is fixed

originFirst attested in the 1660s, probably from cant (“slope, edge, corner”) + lever, but the earliest form (c. 1610) was cantlapper. First element may also be Spanish can (“dog”), an architect's term for an end of timber jutting out of a wall, on which beams rested.

Etymology from Wiktionary, CC BY-SA 3.0