suck
吮 centers on drawing something in with the mouth: sucking, as a baby suckles or a person sucks liquid through a straw.
吮 combines 口 (mouth) with 允, which likely contributes the sound. The exact historical development of 允 in this character is uncertain.
The mouth 口 beside 允 suggests a mouth drawing something in. Picture a baby's mouth sucking milk from a bottle.
For セン, imagine a thirsty person sucking water through a straw and saying 'Ah, refreshing!' — the 'sen' sound in 'refreshing' cues セン.
currying favour (favor); brown-nosing; bootlicking; sucking pus from carbuncles and licking hemorrhoids (haemorrhoids)