red ink (Noun) — The amount by which the cost of a business exceeds its revenue.
red ink (Noun) — A liquid used for printing, writing or drawing.
red ink (Noun) — Dark protective fluid ejected into the water by cuttlefish and other cephalopods.
red ink (Verb) — Mark with red ink. ex. "red-ink the area that is restricted"
red ink (Verb) — Append one's signature to. ex. "They inked the contract"
red ink (Verb) — Mark, coat, cover, or stain with ink. ex. "he inked his finger"
red ink (Verb) — Fill with ink. ex. "ink a pen"