Adam Clarke on Genesis 25:27

A man of the field - איש שדה ish sadeh, one who supported himself and family by hunting and by agriculture.

Jacob was a plain man - איש תם ish tam, a perfect or upright man; dwelling in tents - subsisting by breeding and tending cattle, which was considered in those early times the most perfect employment; and in this sense the word תם tam, should be here understood, as in its moral meaning it certainly could not be applied to Jacob till after his name was changed, after which time only his character stands fair and unblemished. See Gen 32:26-30.