Never used independently in modern Chinese. Its value is as a building block: it is the phonetic in 嗽 (to cough) and 漱 (to rinse the mouth) — both about the mouth and water, picking up the old 'suck / draw in' sense of 欶.
register
Archaic; you only meet it inside other characters and in component notes, never in speech.
The 欠 radical pictures a person with an open, breathing mouth; on the right it tells you the word is about an action of the mouth — here drawing liquid in by sucking.
