An implicit message perceived subtly alongside, but not detracting noticeably from, the explicit message conveyed in or by a book, film, speech or similar (contrast with overtone); an undercurrent.
A pale colour, or one seen underneath another colour.
A low state of the physical faculties.
verb
To accompany as an undertone.
To say or speak in an undertone.
To present as less important, noticeable or prominent.
Men rarely make this mistake, their habitual blunder being to undertone everything, to make too light of Julia's new frock, and Johnnie's symptoms of measles, and the way they waste things down stairs.
2010, Maddy Myers, “Arisia 2010: the Sexiest of Sci-Fi Cons,” thephoenix.com, 26 January, 2010,
It’s hard enough to separate the geeks from the creeps at your average Anime con (it’s more of a Venn Diagram than an either/or, really). The task becomes even more difficult at Arisia, where the sexual undertones are … not particularly undertoned.