one breast
Refers to a single breast, often in contexts where one breast is exposed or affected, as opposed to both.
赤ちゃんが片乳だけ飲んで寝てしまった。
The baby drank from only one breast and fell asleep.
Compound of 片 (kata, 'one side') and 乳 (chi, 'breast'). The reading かたちち is a straightforward combination.