When people talk about altruism, they have in mind the act of being nice to others, being charitable, well-mannered, like giving up your seat for that old lady on the bus.
The person who coined the word altruism – French Philosopher Auguste Comte, used the word to talk about the idea that you have no right to life, liberty or happiness, and all you have are certain unchosen obligations. Comte explains this clearly in his writings – see The Catechism of Positive Religion, Pages. 309, 313, 332–33.
This is what I see often in philosophy, the popular version of the word lends support for the actual meaning lurking beneath, until the opportune moment arrives.