A weak form is an unstressed form of a words or sound, where it is given little prominence in the phrase. Vowel sounds are often reduced to the schwa (uh) when weak.
When we say 'what the hell', 'the' is not an important word, so we give it less importance and it can almost disappear, so we say what-th-hell, all run together into one unit, almost swallowing the definite article- it sounds like a single unit, not three separate words. In spoken English, we run words together a lot.
