cast-iron
Reliable and unlikely to change. Always used before a noun. You can trust me, OK? I only make cast-iron promises.
cast-iron
BRITISH or cast iron
mainly AMERICANA cast-iron guarantee, piece of evidence, etc. is absolutely certain and can definitely be believed. They are demanding cast-iron guarantees of a fair hearing. Even if one could visualize her as a murderer, she has a cast-iron alibi. They will punish you for being late, unless you've a cast-iron excuse.