1.
Someone who is ~ in or ~ on a particular activity is doing that thing. (FORMAL)
They found the three ~ in target practice.
...the various projects he was ~ on.
ADJ: v-link ADJ in/on n
2.
When two people are ~, they have agreed to marry each other.
We got ~ on my eighteenth birthday...
ADJ: usu v-link ADJ, oft ADJ to n
3.
If a telephone or a telephone line is ~, it is already being used by someone else so that you are unable to speak to the person you are phoning. (BRIT; in AM, use busy )
The line is ~...
ADJ: v-link ADJ
4.
If a public toilet is ~, it is already being used by someone else. (BRIT; in AM, usually use occupied )
ADJ: v-link ADJ