The function takes a string input, appends the last character of the input, and then appends the characters "nd".

[CODE]:
```python
def function(input_string):
    return input_string + input_string[-1] + "nd"
```

However, this code might produce an error for empty string input. It is important to handle this edge case:

[CODE_WITH_EDGE_CASE]:
```python
def function(input_string):
    if len(input_string) == 0:
        return "Error: Empty string input is not allowed"
    return input_string + input_string[-1] + "nd"
```