meaning of 'despair so great'

[atabitaraf]

despair so great it threatened to engulf him (one example in Longman contemporary dictionary under the entry 'engulf')

I could not understand the meaning of the above phrase.

 

[Rover_KE]

How does Longman define ‘engulf’?

 

[atabitaraf]

How does Longman define ‘engulf’?

I copy from Longman: 'if an unpleasant feeling engulfs you, you feel it very strongly.'

And the meaning of 'despair so great it threatened to engulf him' is still vague to me.

My problem is more about what 'great' means or refers to here.

 

[jutfrank]

The word great is like 'strong'. The feeling of despair was very strong. He felt as if his despair was going to swallow him up.