word-forming element meaning "under, beneath; less, less than" (in chemistry, indicating a lesser oxidation), from Greek hypo (prep. and adverb) "under, beneath; up from under; toward and under (i.e. into)," from PIE root *upo "under."
1640s, "relating to or characterized by muscular tension," from Greek tonikos "of stretching," from tonos "a stretching," from PIE root *ten- "to stretch." The meaning "maintaining the healthy firmness of tissues" is recorded from 1680s, first extended 1756 to "having the property of restoring to health." Related: Tonical (1580s).
<a href="https://www.etymonline.com/word/hypotonic">Etymology of hypotonic by etymonline</a>
Harper, D. (n.d.). Etymology of hypotonic. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/hypotonic