Albert Barnes Commentary Psalms 129:7

"Wherewith the reaper filleth not his hand, Nor he that bindeth sheaves his bosom." — Psalms 129:7 (ASV)

Wherewith the mower filleth not his hand - It cannot be gathered and stored for the use of cattle, as grass that grows in the field can be. It is valueless for any such purpose, or it is utterly worthless. The phrase filleth not his hand seems to be derived from the idea of reaping, where the reaper with one hand takes hold of the grain which he reaps, and cuts it off with the sickle in the other.

Nor he that bindeth sheaves - This refers to the man who gathers in the harvest. This task was commonly performed by a different person from the reaper.

His bosom - This word would commonly refer to the bosom of the garment, in which things were carried, or that part above the girdle. However, it may be used here in a larger sense. Since it is incongruous to suppose that sheaves of grain would be carried in this way, it likely means simply that the one who gathered the sheaves would usually convey them in his arms, folding them to his bosom.