What does Middle English "bihiȝten" mean?
Solution 1:
behoten = to promise
I find that script hard to read so can't tell about how accurate the ascii is. I searched for a Middle English Dictionary and then tried your text on it and it gave no results. Then I tried other things for the 'y' in 'bihiyten': 'h', 'th', 'g', and then 'gh' which finally worked.
This is plausibly cognate with modern English 'behest'.