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thank [thank 词源字典]: thank: [OE] The notion of ‘gratitude’ in modern English thank arose out of an earlier ‘thoughtfulness’. For the word goes back ultimately to prehistoric Germanic *thank-, *thengk-, which also produced English think, and the noun thank originally meant ‘thought’ (a 12th-century translation of the gospels has ‘From the heart come evil thanks’ Matthew 15:19, where the Authorized Version gives ‘Out of the heart proceed evil thoughts’).

The sense ‘thought’ graduated via ‘favourable thought, good will’ to ‘gratitude’. It was originally singular, and the modern plural usage did not emerge until the 14th century. Thank you first appeared in the 15th century, short for I thank you.
=> think[thank etymology, thank origin, 英语词源]
gravity (n.): c. 1500, "weight, dignity, seriousness, solemnity of deportment or character, importance," from Old French gravité "seriousness, thoughtfulness" (13c.) and directly from Latin gravitatem (nominative gravitas) "weight, heaviness, pressure," from gravis "heavy" (see grave (adj.)). The scientific sense of "downward acceleration of terrestrial bodies due to gravitation of the Earth" first recorded 1620s.
The words gravity and gravitation have been more or less confounded; but the most careful writers use gravitation for the attracting force, and gravity for the terrestrial phenomenon of weight or downward acceleration which has for its two components the gravitation and the centrifugal force. [Century Dictionary, 1902]
thoughtful (adj.): c. 1200, "contemplative, occupied with thought," from thought + -ful. Also in Middle English, "prudent; moody, anxious." Meaning "showing consideration for others" is from 1851 (compare thoughtless.) Related: Thoughtfully; thoughtfulness.