The function takes a string input and replaces the first character with the number of characters in the original string (in words) followed by the digit "8" and the rest of the input string from the second character onwards.

[CODE]:
```python
def f(x):
    num_to_words = {1: "o8ne", 2: "tw8o", 3: "thr8ee", 4: "fo8ur", 5: "fi8ve", 6: "si8x", 7: "se8ven", 8: "ei8ght", 9: "ni8ne", 10: "te8n", 11: "ele8ven", 12: "twe8lve", 13: "thi8rteen", 14: "fo8urteen", 15: "fi8fteen", 16: "si8xteen", 17: "se8venteen", 18: "ei8ghteen", 19:"ni8neteen", 20: "twe8nty"}

    length = len(x)
    if length in num_to_words:
        return num_to_words[length] + x[1:]
    else:
        return "undefined"
```
Please note that this code approximates the behavior of the given function based on available test cases, and it might not be an exact recreation of the original implementation.