A squaddy is an ordinary soldier in the British army — one of the men and women who do the actual marching and fighting, not one of the officers who give the orders. It is British slang, and quite informal. Soldiers use it about themselves and each other, often with rough affection; outsiders using it can sound either friendly or a little dismissive, depending on tone. You meet it in British newspapers, films, and soldiers' own talk, hardly ever in official writing — the army's own word is private, or more formally other ranks. It is also spelt squaddie, and that spelling is the more common one nowadays. The plural is squaddies. An American would not normally say this; they might say grunt or GI instead.