l'accordéon lui touchait le menton !

Hi Sumesh,

It is a particularity of the language to use the definite article with parts of the body often when using reflexive verbs -

Think of the expressions -

Se casser la jambe = to break a leg, literally to 'break oneself the leg' 

Se brosser les dents = To brush your teeth, literally 'to brush oneself the teeth'

Take a look at the following Kwiziq lesson with lots more examples -

https://progress.lawlessfrench.com/revision/grammar/using-le-la-les-with-body-parts-and-clothing-definite-articles

Hope this helps!