the sentence it's from コテッと首をかしげる
「コテッ（と）」 is an onomatopoeia used to describe a sudden and/or quick lean or fall to one side by an object.
'Thus, 「コテッと首{くび}をかしげる」 means "to put one's head on one side". 「コテッと」 would not necessarily need to be translated here.
We have another 「コテッ（と）」 that describes a food item, which means "very oily". These are two different onomatopoeias that happen to be spelt the same.
Ours is a country of mountains, forests, rivers and onomatopoeias.
