thin, emaciated, worn, tired
癯 centers on a state of physical depletion: being thin, emaciated, worn out, or tired, often from illness or hardship.
癯 combines 疒 (sickness) with 瞿, which likely contributes the sound and may relate to a gaunt or staring appearance. The exact historical development is uncertain.
The sickness enclosure 疒 wraps around 瞿, which looks like a pair of staring eyes over a bird-like shape. Picture illness leaving someone so thin and worn that their eyes seem huge and bird-like in a gaunt face.
For ク, imagine a person so emaciated they can only manage a weak 'coo' sound like a tired dove: coo -> ク, and the sickness enclosure shows the cause.