פירוש על ברכות 48:20

WITH ROPES OF VANITY. The prophet bemoans those who draw sin upon themselves through “ropes that are שוא”. The word שוא usually means for naught. Rashi here says that the Gemara means that he will be punished for his even for his inappropriate speech. He did not need to say the sh’ma in the filthy alleyway and even though it is a relatively minor offense as compared to one who sins by action, he will be punished. שוא then refers to speech which is relatively minor as compared to action. Tosfos may not be satisfied with this interpretation because שוא as it is used according to Rashi does not mean for naught, as it usually does, rather, it means for a lesser offense which is not the usual meaning of שוא.
Tosfos therefore offers another explanation. What is meant is – for no reason they draw the sin upon themselves. For it would be better if they did not fulfill the mitzvoh at all rather than saying the words of sh’ma in a place that violates their holiness. The word שוא here means for no positive reason, referring to the motive behind their action when saying the sh’ma in an unclean place. According to this explanation, the word שוא maintains its usual meaning, for naught.