Albert Barnes Commentary Psalms 55:13

"But it was thou, a man mine equal, My companion, and my familiar friend." — Psalms 55:13 (ASV)

But it was you, a man my equal — Margin, “a man according to my rank.” Septuagint, ἰσόψυχε isopsuche — equal-souled, like-souled, “second self” (Thompson); Vulgate, “unanimus,” of the same mind; Luther, “Geselle,” companion.

The Hebrew word used here — ערך ‛êrek — properly means a row or pile, like the showbread piled one loaf on another (Exodus 40:23). Then it would naturally mean one of the same row or pile, or of the same rank or condition.

The word also means price, estimation, or value (Job 28:13; Leviticus 5:15; Leviticus 5:18; Leviticus 6:6).

In this context, the expression may mean a man “according to my estimation, value, or price” — that is, of the same value as myself (Gesenius, Lexicon). More probably, however, it means a man of my own rank, according to my condition: that is, a man whom I esteemed as my equal, or whom I regarded and treated as a friend.

My guide — The word used here properly denotes one who is familiar — a friend — derived from the verb אלף ’âlaph, meaning to be associated with, to be familiar, or to be accustomed to. The noun is frequently used to denote a military leader, the head of a tribe, or a chieftain. In this sense, it is used several times in Genesis 36 to denote the leaders or princes of the Edomites, where it is rendered “duke.” However, here it seems to be used not in the sense of a leader or a guide, but of a familiar friend.

And my acquaintance — The word used here is derived from the verb “to know” — ידע yâda‛ — and its proper meaning is that of “one well known” by us: one who keeps no secrets from us but permits us to understand him thoroughly.

The phrase “my acquaintance” is a feeble expression and does not convey the full force of the original, which denotes a more intimate friend than the word “acquaintance” would suggest. It is language applied to one whom we thoroughly “know,” and who “knows us”; this kind of knowledge exists only in the case of very intimate friends.

All these expressions from the verse would likely be applicable to Ahithophel and to the intimacy between him and David.