Merciless means showing no pity whatsoever. The winning side that will not stop, the interviewer who keeps on asking.
It describes people and forces equally: a merciless dictator, merciless questioning, a merciless sun. Applied to weather or landscape it simply means relentless, with no cruelty intended, since the heat is not out to get you. The word is mercy with -less on the end, and its opposite, merciful, belongs to courts and prayers. The adverb mercilessly usually follows a verb such as teased or beaten.
1adjectivemercilessShowing no pity or forgiveness.
“The critics were merciless.”
2adjectiverelentlessRelentless and unrelieved.
“They walked all day under a merciless sun.”
Also accepted in games: ruthless, pitiless, heartless
Middle English
merciles
English, today
merciless
From Middle English merciles, mercyles, equivalent to mercy + -less.
Etymology from Wiktionary, CC BY-SA