What word does your mother always say?
1886, "below the threshold" (of consciousness), formed from sub "below" (see sub-) + Latin limen (genitive liminis) "threshold" (see limit (n.)).
Apparently a loan-translation of German unter der Schwelle (des Bewusstseins) "beneath the threshold (of consciousness)," from Johann Friedrich Herbart (1776-1841), author of a textbook on psychology published in 1824. The scare over subliminal advertising came in 1957.
subliminal sub·lim·i·nal (sŭb-lĭm'ə-nəl)
Below the threshold of conscious perception. Used of stimuli.
Inadequate to produce conscious awareness but able to evoke a response.