A freezer unit was a technology designed to keep foodstuffs and other perishables in cold storage.
In a whispered aside to Lee Kelso in 2265, Gary Mitchell referred to Doctor Elizabeth Dehner as a "walking freezer unit" when she was unreceptive to his advances. She later revealed she had heard him while visiting him in sickbay. After he apologized, she seemd to agree with his assessment of her behavior, remarking, "Women professionals do tend to overcompensate." (TOS: "Where No Man Has Gone Before")