According to Merriam-Webster, threaten has the following four meanings:
1: to utter threats against
2a: to give signs or warning of
2b: to hang over dangerously
3: to announce as intended or possible
4: to cause to feel insecure or anxious
There is one sentence:
The lecturer's monotonous delivery threatened to put us to sleep.
What does threaten mean in the above sentence?