early 15c., "diligence, industry, activity; anxiety, care, concern," from Old French solicitude (Modern French sollicitude), and directly from Latin sollicitudinem (nominative solicitudo) "anxiety, uneasiness of mind," noun of state from past-participle stem of solicitare (see solicit).
"belonging or relating to the subject or matter in hand," late 14c., from Anglo-French purtinaunt (late 13c.), Old French partenant (mid-13c.) and directly from Latin pertinentem (nominative pertinens) "pertaining," present participle of pertinere "to relate, concern" (see pertain). Related: Pertinently.
The earlier Middle English word was interesse (late 14c.), from Anglo-French interesse "what one has a legal concern in," from Medieval Latin interesse "compensation for loss," noun use of Latin interresse (compare German Interesse, from the same Medieval Latin source).
Financial sense of "money paid for the use of money lent" (1520s) earlier was distinguished from usury (illegal under Church law) by being in reference to "compensation due from a defaulting debtor." Sense of "personal or selfish consideration" is from 1620s. Meaning "business in which several people are interested" is from 1670s. Meaning "curiosity, feeling that something concerns one, appreciative or sympathetic regard" is first attested 1771. Interest group is attested from 1907; interest rate by 1868.
c. 1300, "care, heed," from Latin cura "care, concern, trouble," with many figurative extensions over time such as "study; administration; office of a parish priest; a mistress," and also "means of healing, successful remedial treatment of a disease" (late 14c.), from Old Latin coira-, a noun of unknown origin. Meaning "medical care" is late 14c.
early 14c., perteinen, "be attached legally," from Old French partenir "to belong to" and directly from Latin pertinere "to reach, stretch; relate, have reference to; belong, be the right of; be applicable," from per "through" (from PIE root *per- (1) "forward," hence "through") + tenere "to hold," from PIE root *ten- "to stretch."
From late 14c. as "to belong to as a possession or an adjunct; belong to as one's care or concern," also "have reference to." Related: Pertained; pertaining.
Old English caru, cearu "sorrow, anxiety, grief," also "burdens of mind; serious mental attention," in late Old English also "concern, anxiety caused by apprehension of evil or the weight of many burdens," from Proto-Germanic *karō "lament; grief, care" (source also of Old Saxon kara "sorrow;" Old High German chara "wail, lament;" Gothic kara "sorrow, trouble, care;" German Karfreitag "Good Friday;" see care (v.)).
Meaning "charge, oversight, attention or heed with a view to safety or protection" is attested from c. 1400; this is the sense in care of in addressing (1840). Meaning "object or matter of concern" is from 1580s. To take care of "take in hand, do" is from 1580s; take care "be careful" also is from 1580s.
The primary sense is that of inward grief, and the word is not connected, either in sense or form, with L. cura, care, of which the primary sense is pains or trouble bestowed upon something. [Century Dictionary]