“frustration.” The Hebrew word is kaas (#03708 כַּעַס), and its semantic range includes anger, vexation, provocation, irritation, frustration, grief. One could easily conflate Ecclesiastes 1:18 in English and say, “For in much wisdom is much vexation, anger, grief, and frustration,” and that would be true, because knowing what should be done and the right way to do something is very irritating and frustrating if others are not doing something right, especially if their wrong actions causes hurt or harm. The Theological Wordbook of the Old Testament says “Although the root does not appear in Ugaritic, it is found in Aramaic, Akkadian, and Arabic. The former two emphasize the pain aspect while the Arabic usage stresses sadness and sorrow. Although the root can be used to express physical suffering, it much more commonly has to do with mental anguish.” We decided to go with “frustration” in the REV (cp. Tremper Longman III, The New International Commentary on the Old Testament).
“pain.” The Hebrew word is makob (#04341 מַכְאֹב), and it means mental or physical pain, sorrow, or suffering. We translate it “pain” in the REV (cp. NASB; Rotherham; YLT), but it includes mental pain, sorrow, and suffering.