1550s, "pleasing to the mind," also "full of gratitude, disposed to repay favors bestowed," from obsolete adjective grate "agreeable, pleasant," from Latin gratus "pleasing" (from suffixed form of PIE root *gwere- (2) "to favor"). "A most unusual formation" [Weekley]. A rare, irregular case of English using -ful to make an adjective from an adjective (the only other one I can find is direful "characterized by or fraught with something dreadful," 1580s). Related: Gratefully (1540s); gratefulness.
Grateful often expresses the feeling and the readiness to manifest the feeling by acts, even a long time after the rendering of the favor; thankful refers rather to the immediate acknowledgment of the favor by words. [Century Dictionary]
gwerə-, Proto-Indo-European root meaning "to favor."
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit grnati "sings, praises, announces;" Avestan gar- "to praise;" Lithuanian giriu, girti "to praise, celebrate;" Old Celtic bardos "poet, singer."
"sacrament of the Lord's Supper, the Communion," mid-14c., from Old French eucariste, from Late Latin eucharistia, from Greek eukharistia "thanksgiving, gratitude," later "the Lord's Supper," from eukharistos "grateful," from eu "well" (see eu-) + stem of kharizesthai "show favor," from kharis "favor, grace," from PIE root *gher- (2) "to like, want." Eukharisteo is the usual verb for "to thank, to be thankful" in Septuagint and Greek New Testament. Related: Eucharistic.
c. 1600, "feel pain or distress" (a sense now obsolete); 1620s, "take (something) ill, consider as an injury or affront; be in some degree angry or provoked at," from French ressentir "feel pain, regret," from Old French resentir "feel again, feel in turn" (13c.), from re-, here perhaps an intensive prefix (see re-), + sentir "to feel," from Latin sentire "to feel, think" (see sense (n.)).
It sometimes could have a positive sense in English, "appreciate, be grateful for" (1640s), but this is obsolete. Related: Resented; resenting.
mid-13c., plural of thank (n.), from Old English þanc, þonc in its secondary sense "grateful thought, gratitude," from Proto-Germanic *thanka-, from the same root as thank (v.). In prehistoric times the Germanic noun seems to have expanded from "a thinking of, a remembering" to also mean "remember fondly, think of with gratitude." Compare Old Saxon thank, Old Frisian thank, Old Norse þökk, Dutch dank, German Dank. The Old English noun chiefly meant "thought, reflection, sentiment; mind, will, purpose," also "grace, mercy, pardon; pleasure, satisfaction."
As short for I give you thanks from 1580s; often with extensions, such as thanks a lot (1908). Spelling thanx attested by 1907.