a prefixal use of under, as to indicate place or situation below or beneath (underbrush; undertow); lower in grade or dignity (undersheriff; understudy); of lesser degree, extent, or amount (undersized); or insufficiency (underfeed).
Origin
Middle English;Old English
British Dictionary definitions for under-
under-
prefix
1.
below or beneath: underarm, underground
2.
of lesser importance or lower rank: undersecretary
3.
to a lesser degree than is proper; insufficient or insufficiently: undercharge, underemployed