Meaning "irrational, maniacal, lacking or deprived of mental sense" is from 1520s; meaning "lacking or deprived of moral sense, unfeeling" is from 1550s. Insensate means "not capable of feeling sensation," often "inanimate;" insensible means "lacking the power to feel with the senses," hence, often, "unconscious;" insensitive means "having little or no reaction to what is perceived by one's senses," often "tactless." Related: Insensately; insensateness.
c. 1600, back-formation from stolidity, or else from French stolide (16c.), from Latin stolidus "insensible, dull, slow, brutish, rude, stupid," properly "unmovable," related to stultus "foolish," from PIE *stol-ido-, suffixed form of root *stel- "to put, stand, put in order," with derivatives referring to a standing object or place.
late 14c., "consisting of marble," from marble (n.). Meaning "mottled like marble" is mid-15c. The earlier adjective in this sense was marbrin (early 14c.). From 1590s as "resembling marble in some figurative quality" (cold, hard, insensible, etc.). Marble cake is attested from 1864.
"incapable of relenting, unmoved by pity, unpitying, insensible to the distress of others," 1590s, from relent + -less. Related: Relentlessly; relentlessness. Relenting is from 1590s as a present-participle adjective, "inclined to relent; too easily moved." For a noun for the group, relentment "act or state of relenting" (1620s) has been tried.