Whose nickname was Pim?
"Pim" was the nickname of Otto Heinrich Frank, father of Anne and Margot. Otto was born in Frankfurt am Main in 1889 and grew up to study ecomomics. After serving in the First World War, he worked in his family's bank, and married Edith Hollander in 1925. Together they had two daughters, Margot (born 1926) and Anne (born 1929).
In 1942, the Frank family went into hiding in Amsterdam to avoid capture and punishment by Nazi officials. During this time, Anne kept a diary, which Otto inherited after the war and had published.
In her diary, Anne referred to her father as "Pim," which is a Dutch nickname for people called Willem. Anne presumably gave her father this nickname to create some amount of anonymity or secrecy.