The name MettleCI combines two components:
‘Mettle’ and
‘CI’ - a common abbreviation for ‘Continuous Integration’
Mettle
/ˈmɛt(ə)l/
noun
A person's ability to cope well with difficulties; spirit and resilience.
1: vigor and strength of spirit or temperament
"the team showed their true mettle in the second half"
2: staying quality or stamina
equipment that proved its mettle
proved his mettle in battle
Similar: spirit, fortitude, tenacity, strength of character, moral fibre, steel
CI
In software engineering, continuous integration is the practice of merging all developers' working copies to a shared mainline several times a day.