'Full steam ahead' is an English idiom meaning full speed, full throttle, without hesitation, or all out.
Full steam ahead, sometimes shortened to 'full steam', is a variant on a common phrase of unambiguous meaning: "full speed ahead". In the late 1800s, with the event of steam engines being fitted to ships and trains, one might well hear the cry 'full steam ahead!' used quite literally; as this was the hot new technology of the day, the term became a trendy replacement for 'full speed ahead'.
The phrase appears to have originated on steamships, in which the captain would have to call down to the speaking tube to the ship's engineer tending the boilers to let them know how much power they wanted. The same principles apply to locomotive steam engines, and the phrase was quickly taken up by train engineers as well.
The command 'full steam' quickly fell out of use in shipping, as bridge-to-engineering orders became standardized, and 'full steam' became 'full ahead' in America, and 'full speed ahead/astern' in Commonwealth countries. It is perhaps worth noting that in American terminology, full ahead would result in full speed, but there is one speed even faster, for emergency use: flank speed. Full steam ahead would originally have suggested that you want fast, but not so fast that the boiler might explode if you maintained that speed over a prolonged period of time.