Concept:
Idioms have meanings different from the literal meanings of the words used.
Step 1: Understand the idiom.
The idiom:
\[
\text{beside the mark}
\]
means away from the main point or irrelevant.
Step 2: Check the options.
• Exact → incorrect
• beneficial → incorrect
• irrelevant → correct
• Strange → incorrect
Step 3: Choose the correct meaning.
Thus, the idiom means:
\[
\text{irrelevant}
\]
Step 4: Conclusion.
Hence, the correct answer is:
\[
\boxed{\text{irrelevant}}
\]