Abu Ubaydah said: It is one of the words that bear opposite meanings (1).
Al-Akhfash said: It is as when one says, “This matter is behind you,” meaning that it will come to you; or, “I am behind so-and-so,” meaning that I will reach him (2).
Muqatil said: “Ahead of him is Hell” means: after him (3).
“And he will be given to drink of festering water”: that is, water which is itself festering fluid—the pus and blood that flows from the bodies of the disbelievers (4).
Muhammad ibn Kab said: It is that which flows from the private parts of the fornicators, and the disbeliever will be made to drink it (5).
“He will sip it, but will scarcely be able to swallow it. Death will come to him from every direction, yet he will not die; and ahead of him lies a severe punishment” (17).
“He will sip it”: that is, he will take it in and drink it, not all at once, but sip by sip, because of its bitterness and heat. “But will scarcely be able to swallow it”: the word “scarcely” is supplementary; that is, he will not be able to swallow it, as in His saying: “He could not see it” (al-Nur 24:40), meaning: he did not see it.
Ibn Abbas said: He cannot make it pass down his throat.
It is also said that its meaning is: he can hardly swallow it, yet he does swallow it, and it boils within him.