弩 means "to bulge or protrude" here. In both middle and modern Chinese the homophone 努 is decisively more prevalent for this meaning:
This particular usage of 弩 (that is, used of one's eyes) seems nowhere to be found other than in 老舍's writing, but it is not without motivation. Chinese homophones that look alike often involve interchanging and borrowing in their meanings (通假) to some extent. In the case of 弩 and 努, the former appears to have acquired some of the latter's meanings by the late imperial period, apparently by mistake in the double-syllable word 努力:
Then maybe folk etymology decided that this usage is worthy of more exposure because, say, who could deny that "crossbow" pertains to "bulging"? Anyway, the result is that 弩 broke away from the shackles of one fixed word into more flexible usage by Qing dynasty:
- 弩 is a verb in the context provided. It means "to bulge or protrude".
- Most of the time it comes together with 嘴 or 眼.
- If you want to use it, write 努.