Few in Westeros could compare to Petyr Baelish in the art of manipulation and intrigue. Born into a family with neither power nor vast lands, he managed to break into the upper echelons of nobility, gradually accumulating influence. He was trusted to manage the royal treasury, he knew how to negotiate with those stronger than him and change the balance of power in his favor. But even as his power grew, he always had one nickname "Littlefinger". Why?
At first glance, this might seem like an insignificant detail, but in fact the name reflected his path and position in the world of great houses. The nickname appeared in Baelish's youth, when he was raised in the Tully house.
Despite his origins, Petyr dreamed of the impossible, he was madly in love with Catelyn Stark, then still Tully, and tried to win her hand. His claims seemed presumptuous and even ridiculous to the representatives of a powerful house, for he was only the son of a humble lord from a tiny land on the Fingers, the rocky shores near the Trident.

The nickname "Littlefinger" was given to him by Catelyn's brother, Edmure Tully. Firstly, because of Petyr's short stature, and secondly, in mockery of his ancestral lands, tiny Fingers, which could hardly compare with the holdings of great lords.
Thus, young Baelish became "Littlefinger", and this name remained with him forever. It emphasized his origins, but over time it became a symbol of a man who, despite his humble beginnings, managed to change the course of history.