impalpable adj.

word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."

In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.

late 14c., "that can be felt, perceptible by the touch," from Late Latin palpabilis "that may be touched or felt," from Latin palpare "touch gently, stroke," a word de Vaan finds to be of no known etymology (rejecting the connection in Watkins, etc., to a reduplication of the PIE root *pal-, as in feel (v.), on phonetic grounds). Some sources suggest it is onomatopoeic. The figurative sense of "easily perceived, evident, clear, obvious" also is from late 14c., on the notion of "seeming as if it might be touched." Related: Palpably; palpability.