Learn Portuguese (Portugal): Quando preciso do dicionário, peço-lho. - When I need the dictionary, I ask him for it.

-lho combines two pronouns: the indirect lhe (to him) and the direct o (it). The o part refers back to o dicionário (masculine, singular), while the lhe part tells us who is being asked.

Does -lho always mean "him"? Could I use this exact sentence to mean "I ask HER for it"?

Yes, you can use the exact same sentence! The pronoun lhe can mean "to him", "to her", or even formal "to you". So -lho can refer to any of those people, depending on the context of the conversation.

Why do we say preciso do instead of just preciso o?

The verb precisar (to need) is always followed by the preposition de when you state what you need. Here, de merges with the word o (the) to form the contraction do before dicionário.

Why does -lho attach to the end of peço with a hyphen?

In European Portuguese, object pronouns typically attach to the end of the verb in an affirmative statement. Because the comma acts as a pause, peço is treated as the first word of its clause, so the pronoun must come directly after it.