We all had nicknames at school, my most unfortunate one was "Slug", not because a silverly slimy trail came out of my bottom as I squelched my way to school on a single muscular foot, but because it evolved from my family nickname "Lugs", which is now not very often used. They've got used to calling me Mel. Another chap (hello Jim) was known as "Blockhead" or "Tefal Head" an eighties phenomenum from an advert with a boffin with an exceptionally large forehead. See this link for what I mean.
There were others with equally cruel head-based nicknames, being known as slug was quite unique it seems. Other nicknames from my school classmates, "onion head", "bean head" as well as some others I fail to recall.
When I was at uni, I spoke to someone about this nicknaming lark, specifically fixating itself on heads, and he confided in me that his nickname at school was "train head". Which was hysterically surreal. Sure, you can associate someone's head with the shape of a vegetable and such, but a train? But then I studied his face. It had the appearance of the age of steam, all earnest and angular and motoring along on a single track, his features juddering away as he purposely motored forward. He was a train head all right, I wanted to shout "Woo Woo!" and pull an imaginary train drivers whistle thing as he walked, there was no doubt about it, he was a train head. FUCKING TRAIN HEAD! YOU'RE SHIT TRAIN HEAD! TRAIN HEAD! WOO WOO! CLIVE IS A TRAIN HEAD! I was screaming this in my mind and bouncing round him like a gibbon pointing at him, but in the real world I just remarked calmly. "Oh children, they can be so cruel, I can't see the resemblance myself".
I don't think I ever spoke to him again, fucking TRAIN HEAD!