The term 'strong string' can refer to three different items: CORD (4 letters), TWINE (5 letters), and catgut (6 letters). CORD is a thick, strong rope made of several strands of material twisted together. TWINE is a thin, strong string made of two or more strands of material twisted together. Catgut is a strong string made from the intestines of sheep or goats. All three of these items are commonly used for tying, binding, and other purposes.