For the Egyptians shall help in vain - That is, if they enter into the alliance, they shall not be able to defend you from the invader. The other member of the sentence would seem 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 have I cried - Therefore have I the prophet cried, that is, I do call her so.
Concerning this - Concerning this country; that is, Egypt. Some have understood this as referring to Jerusalem, but the connection requires us to understand it of Egypt.
Their strength is to sit still - This is evidently designed to be an expressive appellation of Egypt. The word rendered here, without much propriety, ‘strength’ (רהב râhab) is a proper name of Egypt, and is several times applied to it; 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 in Psalms 87:4; Psalms 89:10 (see the margin). Why it was given to Egypt is unknown, and can only be conjectured. Bochart (“Geog. Sacra,” i. 4. 24) supposes that it is derived from the word ῥιβι ribi, which singifies “a pear,” and that it was given to the Delta or Lower Egypt on account of its form, as 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 forsake the usual sense of the Hebrew word. The verb רהב râhab means to urge, press on, attack Proverbs 6:3; to be highspirited, fierce, full of courage; to behave proudly Isaiah 3:5; and has, in most instances, a relation to pride, to arrogance, to boasting Job 9:13; Psalms 40:4. The noun “Rahab” indicates ferocity, haughtiness, boasting, 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 rendered ‘to sit still,’ is a part of the name which the prophet gave to her. Though she boasted, yet would she sit still; she would be inefficient, and would do nothing; and the whole name, therefore, may be rendered, ‘I call her, the blusterer that sitteth still;’ that is, ‘they are courageous in talking; cowards in acting.’ (Taylor)