This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
唍 combines 口 (mouth) with 完, which likely contributes the sound. The exact historical development is uncertain.
With 口 (mouth) beside 完, imagine a mouth completing a smile, but the true meaning is unclear.
For カン, imagine a can (カン) being opened with a mouth-like opener: can -> カン, and the mouth component 口 suggests the sound.