Neglect: This implies a complete lack of effort or attention.
Avoidance: This suggests deliberately steering clear of something.
Indifference: This indicates a lack of interest or concern.
Abandonment: This means giving up completely.
Laziness: This refers to a general unwillingness to work or exert oneself.
Passive: This is the opposite of actively trying and suggests being inactive or unresponsive.
It's also worth noting that the opposite of "endeavor" might be something like success or achievement, depending on the specific situation.
For example, "She endeavored to climb the mountain, but her success was thwarted by the storm."
Ultimately, the best antonym for "endeavor" will depend on the specific context of its use.