yâsaph (יָסַף , Strong's #3254), “to add, continue, do again, increase, surpass.” This verb occurs in the northwest Semitic dialects and Aramaic. It occurs in biblical Hebrew (around 210 times), post-biblical Hebrew, and in biblical Aramaic (once).
Basically, yâsaph signifies increasing the number of something. It may also be used to indicate adding one thing to another, e.g., “And if a man eat of the holy thing unwittingly, then he shall put the fifth part thereof unto it, and shall give it unto the priest …” (Lev. 22:14).
This verb may be used to signify the repetition of an act stipulated by another verb. For example, the dove that Noah sent out “returned not again” (Gen. 8:12). Usually the repeated action is indicated by an infinitive absolute, preceded by the preposition le — “And he did not have relations with her again.”
Literally, this reads “And he did not add again [‛od] to knowing her [intimately]” (Gen. 38:26).
In some contexts yâsaph means “to heighten,” but with no suggestion of numerical increase. God says, “The meek also shall increase [yâsaph] their joy in the Lord …” (Isa. 29:19). This same emphasis appears in Ps. 71:14: “… and will yet praise thee more and more [yâsaph]’ or literally, “And I will add to all Thy praises.” In such cases, more than an additional quantity of joy or praise is meant. The author is referring to a new quality of joy or praise — i.e., a heightening of them.
Another meaning of yâsaph is “to surpass.” The Queen of Sheba told Solomon, “Thy wisdom and prosperity exceedeth the fame which I heard,” or literally, “You add [with respect to] wisdom and prosperity to the report which I heard” (1 Kings 10:7).
This verb may also be used in covenantal formulas, e.g., Ruth summoned God’s curse upon herself by saying, “The Lord do so to me, and more also [yâsaph], if ought but death part thee and me,” or literally, “Thus may the Lord do to me, and thus may he add, if …” (Ruth 1:17; cf. Lev. 26; Deut. 27-28).