Albert Barnes Commentary Isaiah 30:7

"For Egypt helpeth in vain, and to no purpose: therefore have I called her Rahab that sitteth still." — Isaiah 30:7 (ASV)

For the Egyptians will help in vain - That is, if they enter into the alliance, they will not be able to defend you from the invader. The other part of the sentence seems to imply that they would make promises of aid, and would even boast of being able to deliver them, but that they would fail in their promises.

Therefore I have cried - Therefore I, the prophet, have cried; that is, I call her so.

Concerning this - This refers to this country; that is, Egypt. Some have understood this as referring to Jerusalem, but the context requires us to understand it as referring to Egypt.

Their strength is to sit still - This is clearly intended to be an expressive name for Egypt. The word translated here, somewhat inappropriately, as ‘strength’ (רהב râhab) is a proper name for Egypt, and is applied to it several times (Isaiah 51:9):

Art thou not it that hath cut Rahab
And wounded the dragon?

In this passage, there can be no doubt that it refers to Egypt. So also in Psalm 87:4 and Psalm 89:10 (see the margin). Why this name was given to Egypt is unknown and can only be conjectured.

Bochart (in “Geog. Sacra,” i. 4. 24) supposes that it is derived from the word ῥιβι ribi—which signifies “a pear”—and that it was given to the Delta or Lower Egypt on account of its form, somewhat resembling a pear. But there is not clear evidence that such was the meaning of the word, and there is no reason why we should abandon the usual sense of the Hebrew word.

The verb רהב râhab means to urge, press on, attack (Proverbs 6:3); to be high-spirited, fierce, full of courage; to behave proudly (Isaiah 3:5); and, in most instances, relates to pride, arrogance, or boasting (Job 9:13; Psalms 40:4). The noun “Rahab” indicates ferocity, haughtiness, boasting, and insolence; and the name was doubtless given to Egypt on account of its insolence and pride.

It is used here because Egypt would be full of self-confidence and would boast that she could aid the suppliant Jews and deliver them from the threatened invasion. The phrase translated ‘to sit still’ is a part of the name which the prophet gave to her. Though she boasted, yet she would sit still; she would be inefficient and would do nothing. The whole name, therefore, may be translated, ‘I call her, the blusterer that sits still;’ that is, ‘they are courageous in talking; cowards in acting.’ (Taylor)