means The state of being tired — that physical or mental weariness signalling a need for rest.

from Built from the adjective "tired" plus the noun-making suffix "-ness," which English has long used to turn qualities into things you can name (darkness, kindness, sadness). "Tired" itself comes from the Old English verb "tēorian," meaning to fail, grow weary, or come to an end — so to be tired is, at root, to be running out.