underword

Etymology
From.

Noun

 * 1) A word placed below another, either literally or metaphorically: a subtext; an inadequate or subordinate term or description; a secret word or phrase; a watchword
 * 2) * 1867, The loyalist's daughter, by a royalist - Page 8:
 * The soldier hesitated, and seemed to mistrust the party; he raised his lantern, and cast a scrutinising look at them, but an underword and a mysterious look of recognition removed his doubts, and they passed on.