Describes something you do or give with all of yourself in it — no part of you holding back, no private doubts kept in reserve. It goes most naturally with things like support, agreement, approval, welcome, effort, and apology: 'her whole-hearted support', 'a whole-hearted attempt'. The idea underneath is that a person can agree with only part of their heart — politely, or because they must — and this word says that did not happen here. It is a warm, positive word, slightly formal, common in writing and in speeches; in ordinary chat people are more likely to say 'totally behind you' or 'really meant it'. Note that it describes the giving of yourself, so it sits in front of nouns about actions and feelings, not in front of people — we do not usually say 'a whole-hearted man'. It is written with a hyphen or as one word, 'wholehearted'; both are accepted and the one-word form is now more common in American writing.