Evalina writes that when “the fop” took her hand, she pulled it away, but she “could scarce forbear laughing.” By this, she means that she

This is the passage you are asking about; it is found in Letter XI.

Not long after, a young man, who had for some time looked at us with a kind of negligent impertinence, advanced on tiptoe towards me; he had a set smile on his face, and his dress was so foppish, that I really believed he even wished to be stared at; and yet he was very ugly.

Bowing almost to the ground with a sort of swing, and waving his hand, with the greatest conceit, after a short and silly pause, he said, "Madam-may I presume?"-and stopt, offering to take my hand. I drew it back, but could scarce forbear laughing. "Allow me, Madam," continued he, affectedly breaking off every half moment, "the honour and happiness-if I am not so unhappy as to address you too late-to have the happiness and honour-"

Again he would have taken my hand; but bowing my head, I begged to be excused, and turned to Miss Mirvan to conceal my laughter. He then desired to know if I had already engaged myself to some more fortunate man? I said No, and that I believed I should not dance at all. He would keep himself he told me, disengaged, in hopes I should relent; and then, uttering some ridiculous speeches of sorrow and disappointment, though his face still wore the same invariable smile, he retreated.

this is the definition of 'fop' from the Mirriam- Webster Dictionary.

Definition of FOP

1obsolete : a foolish or silly person

2: a man who is devoted to or vain about his appearance or dress : coxcomb, dandy

Evalina is by no means attracted to the fop. She speaks about his dress being so foppish that he must 'wish' to be stared at. Fops (dandies) are usually quite preoccupied with their clothes and vain about their appearance; that she questions his dress and believes him desirous of attention say to me that he wore ruffles, a wig, and powdered his face. She couldn't take him seriously and all she could do was look away so he wouldn't see her laugh at him.

Picture yourself at an important function; prom, or something you'd need to really dress up for............ then picture someone dressed so over the top that you'd have to keep yourself from staring.......... what do you think afterwards? That they want your attention. Do you laugh at their clothing? Well, I'm going to give you the benefit of the doubt and believe that like Evalina you try yo hide your amusement!