underthighEtymologyFrom under + thigh.nounThe back of a person's thigh.“1883, Albert Ellery Berg (ed.), The Universal Self-Instructor, New York: Thomas Kelly, “Out-Door Exercises,” p. 350, You may walk so as to use neither calf, under-thigh, nor hip to any appreciable extent; but it is hard work to run in any way so as not to develop all three […]”The back upper portion of a pantleg or trouser leg.“[…] you will, I hope, never omit taking the hip-measure, which will produce a proportionate under-thigh: you will find at all times the under-sides, from the crutch to the hip, longer than the top sides.”