Mettlesome means full of energy and fight — ready, spirited, not easily tired or scared. You will mostly meet it in books, not in the school corridor. Writers like it for horses that are keen and lively, and for people who have guts. The core is mettle: your spirit, your courage, the “stuff” you are made of when things get hard. We still say someone showed their mettle. Add -some and you get “full of that spirit”. Though actually, mettle looks like metal, and that is not only a spelling accident: they were once the same word, and the picture is that a person’s character is the metal they are made from. Today the two spellings have different jobs, and mettlesome always goes with spirit, never with iron or gold. It is old-fashioned and a little literary. A kind modern substitute is spirited, plucky, or full of fight.

精力旺盛的、勇於向前的。偏書面；寫馬時也指有精神、烈。