expression, noun-modifying (irregular)
in heat; estrous
Fixed expression used for female animals in estrus. The phrase is a prenominal modifier, typically followed by a noun like 犬 (dog) or 猫 (cat).
See also: 盛り (さかり)
盛りのついた猫が夜中にうるさく鳴いている。
A cat in heat is meowing loudly in the middle of the night.
盛りのついた犬は落ち着きがなくなる。
A dog in heat becomes restless.
盛り (さかり) is the noun meaning 'heat' or 'estrus' for animals; 盛りのついた is the prenominal form meaning 'in heat'.
From 盛り (さかり, 'heat' or 'estrus') + の (possessive particle) + ついた (past tense of つく, 'to be in a state'). Literally 'having entered heat'.