English word thoughtless comes from English -less (Lacking (something); without (something).), English thought.
Thoughtless detailed word origin explanation
|-less || English (eng) ||Lacking (something); without (something). |
|thought || English (eng) ||(uncountable) The process by which such forms arise or are manipulated; thinking.. A way of thinking (associated with a group, nation or region).. Form created in the mind, rather than the forms perceived through the five senses; an instance of thinking. |
|thoughtless || English (eng) ||Inconsiderate, inattentive.. Lacking thought or consideration.. Marked by or showing lack of due thought or care; careless. |
Words with the same origin as thoughtless