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 wasn't receptive to his advances.
She later revealed she had heard him while visiting him in sickbay. After he apologized, she seemed to agree with his assessment of her behavior, remarking, "Women professionals do tend to overcompensate." (TOS: "Where No Man Has Gone Before")
In the first draft of the script, [Clark] Mitchell had described Dehner as "Frigid Liz" to Spock, who dryly replied that "The human mechanism is capable of generating a surprising amount of energic heat – depending on the skill of the operator." Also in this draft, Mitchell amended his later apology to Dehner with "But you've got to admit you wear a pretty frosty exterior."
In the final draft of the script for "Where No Man Has Gone Before", he instead referred to her as a "walking refrigerator."