John Gill Commentary Numbers 5:19

"And the priest shall cause her to swear, and shall say unto the woman, If no man have lain with thee, and if thou have not gone aside to uncleanness, being under thy husband, be thou free from this water of bitterness that causeth the curse." — Numbers 5:19 (ASV)

And the priest shall charge her by an oath
Or give her her oath:

and say unto the woman, if no man hath lain with thee :
besides her husband:

and thou hast not gone aside to uncleanness [with another] instead of
thy husband ;
which is but another phrase expressive of the same thing, the sin of adultery:

be thou free from this bitter water that causeth the curse ;
if this is the case, it shall produce no bitter effects, or bring any curse upon thee.