He sighed deeply in his spirit ( ἀναστενάξας) Another graphic touch of this evangelist; such as he had learnt in all probability from St. Peter. The word occurs nowhere but here. It is the outcome of grief and indignation, in which, however, grief predominates.
There shall no sign be given unto this generation ( εἰ δοθήσεται σημεῖον). This is a Hebrew idiom, based upon a form of taking an oath which prevailed amongst the Jews. The full form would be, "God do so and so to me, if so and so."
Hence the hypothetical part of the clause came to be used alone, expressing a very strong form of denial or refusal.