c. 1300, "fixed payment (usually in exchange for taxes collected, etc.), fixed rent," from Old French ferme "a rent, lease" (13c.), from Medieval Latin firma "fixed payment," from Latin firmare "to fix, settle, confirm, strengthen," from firmus "strong; stable," figuratively "constant, trusty" (from suffixed form of PIE root *dher- "to hold firmly, support").
Sense of "tract of leased land" is first recorded early 14c.; that of "cultivated land" (leased or not) is 1520s. A word of confused history, but there is agreement that "the purely agricultural sense is comparatively modern" [Century Dictionary]. There is a set of Old English words that appear to be related in sound and sense; if these, too, are from Latin it would be a very early borrowing. Some books strenuously defend a theory that the Anglo-Saxon words are original (perhaps related to feorh "life").
Phrase buy the farm "die in battle," is from at least World War II, perhaps a cynical reference to the draftee's dream of getting out of the war and going home, in many cases to a peaceful farmstead. The simple term buy it as slang for "suffer a mishap," especially "to die" is attested by 1825, and seems to have been picked up in airmen's jargon. Meanwhile fetch the farm is prisoner slang from at least 1879 for "get sent to the infirmary," with reference to the better diet and lighter duties there.
Proto-Indo-European root meaning "to hold firmly, support."
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dharmah "custom, statute, law," dharayati "holds;" Prakrit dharaṇa "a holding firm;" Iranian dāra‑ "holding;" Greek thronos "seat;" Latin firmus "strong, steadfast, enduring, stable;" Lithuanian diržnas "strong;" Welsh dir "hard," Breton dir "steel."
c. 1300, herber, "herb garden, pleasure garden," from Old French erbier "field, meadow; kitchen garden," from Latin herba "grass, herb" (see herb). Later "a grassy plot" (mid-14c., a sense also in Old French), "shaded nook, bower formed by intertwining of trees, shrubs, or vines" (mid-14c.). It is probably not from Latin arbor "tree" (see arbor (n.2)), though perhaps that word has influenced its spelling:
[O]riginally signifying a place for the cultivation of herbs, a pleasure-ground, garden, subsequently applied to the bower or rustic shelter which commonly occupied the most conspicuous situation in the garden ; and thus the etymological reference to herbs being no longer apparent, the spelling was probably accommodated to the notion of being sheltered by trees or shrubs (arbor). [Hensleigh Wedgwood, "A Dictionary of English Etymology," 1859]