(1) A person properly described as an aggressive or hostile environmentalist; one who displays misanthropic tendencies in support of "Green" causes, or one who takes their beliefs and practices to an antisocial extreme; (2) One who is mentally unbalanced, cognitively or emotionally, due to environmental factors; an Anagram of "environmentalist"
The favorite habitats of most "mentalenvironists" seem to be the chairs in front of their computers, rather than actually outside with bugs and pests, and plants, and dirt, and so forth.
The act of thinking something through in your mind, usually a phrase or sentence, before actually saying it. This is usually advised by someone when a person starts spurting out random thoughts that make absolutely no sense, and add no value to the current conversation.