mask and then fillna
a b c
1 1 5.0
1 1 None
1 1 4.0
1 2 1.0
1 2 1.0
1 2 4.0
2 1 3.0
2 1 2.0
2 1 None
2 2 3.0
2 2 4.0
mask = (df['a']==1) & (df['b']==1)
mean = df.loc[mask, 'c'].mean()
df.loc[mask, 'c'] = df.loc[mask, 'c'].fillna(mean)
df['c'] = df['c'].mask(mask, df['c'].fillna(mean))
#similar
#df['c'] = np.where(mask, df['c'].fillna(mean), df['c'])
print (df)
a b c
0 1 1 5.0
1 1 1 4.5
2 1 1 4.0
3 1 2 1.0
4 1 2 1.0
5 1 2 4.0
6 2 1 3.0
7 2 1 2.0
8 2 1 NaN
9 2 2 3.0
10 2 2 4.0