朽 is classical/literary in standalone use. In modern Chinese it appears mainly in compounds: 腐朽 (decadent, rotten), 不朽 (immortal, undying), 老朽 (decrepit, feeble old person — self-deprecating).
木 (wood) is the indexing radical. The original image is decaying timber — a fallen tree softening into rot. 朽 keeps this primary sense (rotten wood) and extends to anything that decays with age. Compare 枯 (withered), 腐 (rot — flesh-side), 蛀 (worm-eaten).
丂 (kǎo) supplies the sound, drifted to xiǔ. 丂 rarely appears today — it's an archaic graph for halting breath, and survives mostly as the phonetic skeleton inside 考 (test), 巧 (skilful), 朽 (rotten) and 号 (number). A useful spotter character.
