Albert Barnes Commentary Mark 8:12

"And he sighed deeply in his spirit, and saith, Why doth this generation seek a sign? verily I say unto you, There shall no sign be given unto this generation." — Mark 8:12 (ASV)

Sighed deeply in his spirit. His soul, his heart, was deeply affected at their wickedness and hypocrisy. The word spirit, here, means human soul. He drew groans deeply from his breast.

No sign be given. That is, no such sign as they asked—namely, a sign from heaven. He said a sign should be given, the same as was provided by Jonah (Matthew 16:1). But this was not what they asked, nor would it be given because they asked for it.